1. Field of the Invention
The present invention relates to DC-to-DC power converter circuits, and more particularly, to a power converter having a self-driven synchronous rectifier circuit for a non-optimal reset secondary voltage that remains at a zero voltage level during a portion of a switching cycle.
2. Description of Related Art
Self-driven synchronous rectification circuits are known in the art for providing rectification of a voltage that alternates between positive and negative values in a DC-to-DC power converter circuit. An example of a conventional self-driven synchronous rectification circuit is provided in FIG. 1.
More specifically, the self-driven synchronous rectification circuit of FIG. 1 is provided on the secondary side of a transformer 20 having a primary winding 22 and a secondary winding 24. The self-driven synchronous recification circuit includes first and second rectifiers 12, 14 that are each provided by MOSFET devices, e.g., n-channel enhancement-type MOSFETs. The first rectifier 12 has a drain terminal connected to a first end A of the transformer secondary winding 24 and the second rectifier 14 has a drain terminal connected to a second end B of the transformer secondary winding. The gate terminal of the first rectifier 12 is connected to the second end B of the transformer secondary winding through a current limiting resistor 16 and to ground through resistor 17. The gate terminal of the second rectifier 14 is connected to the first end A of the transformer secondary winding through a current limiting resistor 18 and to ground through resistor 19. The source terminals of the first and second rectifiers 12, 14 are coupled to ground. The synchronous rectification circuit provides an output voltage (V.sub.OUT) between a positive terminal and ground. The positive terminal is coupled to the second end B of the transformer through output storage choke 26. A capacitor 28 is coupled between the positive terminal and ground to filter high frequency components of the rectified output voltage (V.sub.OUT).
The operation of the self-driven synchronous rectification circuit of FIG. 1 is illustrated with respect to the timing diagram of FIG. 2a, which depicts the voltage between the B and A ends of the secondary winding of the transformer (V.sub.B-A). In FIG. 2a, the voltage V.sub.B-A is illustrated as a series of rectangular pulses having a predetermined duty cycle that alternates between a positive voltage and a negative voltage. When the voltage V.sub.B-A is positive, i.e., the voltage at end B is positive with respect to the voltage at end A, the first rectifier 12 is turned on and the second rectifier 14 is turned off. This causes a current path to form through the first rectifier 12, the transformer secondary winding 24, and the storage choke 26 to deliver power to the output terminals. Conversely, when the voltage V.sub.B-A is negative, i.e., the voltage at end B is negative with respect to the voltage at end A, the first rectifier 12 is turned off and the second rectifier 14 is turned on. This causes a path for magnetization current stored in the choke 26 during the previous part of the cycle through the second rectifier 14 and the storage choke 26 to the output terminals.
Power is delivered to the secondary side of the transformer only during the positive part of the cycle. The negative part of the cycle is used to reset the transformer. The first rectifier 12 is generally known as the "forward" synchronous rectifier since it is used to conduct current to the output terminals from the transformer 20 during the positive part of the power cycle. The second rectifier 14 is generally known as the "free-wheeling" synchronous rectifier since it is used to conduct current to the output terminals during the negative part of the cycle while the transformer 20 is resetting. When operated with the secondary voltage depicted in FIG. 2a, the gate drives of the rectifiers 12, 14 are synchronized with current flow through the body diodes of the MOSFET devices. In other words, very little current flows through the body diodes of the MOSFET devices when the secondary voltage has an "optimum reset" waveform in the form of FIG. 2a.
A significant drawback of the self-driven synchronous rectification circuit of FIG. 1 is that its efficiency is substantially degraded when MOSFET devices are driven by a "non-optimal reset" secondary voltage across the transformer 20. FIG. 2b depicts a "non-optimal reset" secondary voltage waveform in which the voltage V.sub.B-A remains at the zero level during a portion of one switching cycle. Specifically, the zero voltage state occurs after the negative voltage state (reset) and before the next positive voltage state. When the secondary voltage V.sub.B-A is zero, both the first rectifier 12 and the second rectifier 14 are turned off. Magnetization current of the storage choke 26 is conducted through the body diode of the second rectifier 14 during the zero voltage portion of the switching cycle. It is undesirable for the body diodes of the MOSFET devices 12, 14 to conduct current during a substantial portion of the switching cycle since they cause a voltage drop that results in substantial power loss, i.e., reduced efficiency.
Accordingly, it would be desirable to provide a power converter having a self-driven synchronous rectification circuit that overcomes these and other disadvantages of the prior art.